Coral colonies are made up of hundreds or even thousands of genetically identical individuals called polyps. These polyps have microscopic algae called zooxanthellae living within their tissues. The zooxanthellae work like an internal symbiotic vegetable garden, carrying out photosynthesis and providing nutrients which help reef-building corals create reef structures. When a coral bleaches, it loses its zooxanthellae, and will die within a matter of weeks unless the zooxanthellae can be replaced. The term bleaching is used because the dazzling colors of living corals are due to the colors of zooxanthellae in coral tissue, and when zooxanthellae are lost, corals appear white, or "bleached."

Coral bleaching is not well understood by scientists. Many different hypotheses exist as to the cause behind coral bleaching, but the strongest evidence points to unusually warm sea surface temperatures as being the main factor (Glynn 1993). Coral bleaching events worldwide have been attributed to sea surface temperatures (SSTs) rising and staying as little as 1°C higher than the usual average monthly maximum SST during the hottest months of the year (Goreau and Hayes 1994). In Jamaica, significant coral bleaching and death occur when SSTs remain at 29.3°C or higher for one month (Hoegh-Guldberg 1999). Therefore, in the Caribbean and Florida Keys, when SSTs rise and stay above this thermal threshold, coral bleaching is likely to occur.

Corals polyps are beautiful living animals that are enjoyed by millions of snorkelers and divers world wide, but they are also of vital importance the whole coral reef ecosystem that they live upon. The corals themselves are the main building blocks of coral reefs, building these massive structures bit by bit through the construction of their skeletons. If corals cannot survive and multiply, reefs will be lost through natural physical and biological erosion as waves, predators, and other factors affect corals.

The death of corals and the resulting disappearance of reefs would result in the loss of an uncountable number of marine invertebrates and fishes that rely on the physical structure of the reef for survival. Coral reefs are home to hundreds of thousands of species, including many not yet known to science. The elimination of coral reefs would also result in great losses of income and resources from tourism, fishing, and the discovery of new medicines (Carte 1996). The reefs of the Florida Keys alone account for 1.6 billion dollars of revenue annually (Birkeland 1997). Coral reefs also protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion and flooding.

Until the 1980's, the only coral bleaching event recorded was due to flooding from Hurricane Flora that resulted in a large drop in salinity that bleached and killed many corals in Jamaica (Goreau 1964). Mass coral bleaching was first recognized on the Pacific coast of Panama following the 1982-83 El Nino event (Glynn 1984). The warm SSTs associated with the El Nino event were identified as the cause of death of over 99% of corals and the complete loss of reef structures in the Galapagos Islands and the death of over 50% of corals in Panama (Glynn and D’Croz 1990; Glynn 1993). The sensitivity of corals to small temperature changes then became a major concern of researchers as predictions of global warming and stronger, farther-reaching El Nino events came to light. The 1997-98 El Nino event is the strongest on record to date, resulting in unprecedented coral bleaching and death across the globe (Wilkinson et al. 1999). SSTs are expected to continue to increase worldwide (Hoegh-Guldberg 1999) and El Nino events are expected to increase in frequency, strength and duration, endangering the future survival of coral reefs.

It has been shown that stress caused by pollution, solar radiation, changing salinity, and bacteria can also result in coral bleaching. The Coral Reef Early Warning System (CREWS) implemented by the National Oceanic and Atmospheric Administration (NOAA) considers various physical factors and issues ‘bleaching alerts’ when coral bleaching appears likely. The CREWS software evaluates the intensity of solar radiation, wind speed and sea surface temperatures (SSTs) when issuing coral bleaching alerts.

Wind speed and surface temperature must be considered together because low wind speeds allow greater amounts of solar energy to penetrate the smoother water surface, thus raising the temperature. Also, when corals are exposed to very high sunlight combined with low wind speed, the zooxanthellae may become photodamaged, making a bleaching response more likely. Using CREWS buoy data near coral reefs helps give scientists a heads up so that they can adequately monitor and study these events. During the 1997-98 El Nino event, the effectiveness of this system was shown as bleaching was predicted and occurred on Sombrero Reef in the Florida Keys (Hendee et al. 2001). It is currently impossible for us to prevent bleaching of corals, but through the use of the CREWS buoy data we can at least predict and learn more about this destructive process.

References

Birkeland C (ed) (1997) Life and death of coral reefs. Chapman and Hall, New York.